1. Field of the Invention
The present invention relates to a computer system, a storage subsystem, and a write processing control method.
2. Description of the Related Art
The performance of the storage subsystem has been improved by effectively utilizing a cache. For example, the storage subsystem holds data likely to be reread very soon in the cache, thereby the data can be read at high speed when the data is reread. Moreover, by reporting the end of write operation for written data from server as soon as it is stored into the cache, even when the storage subsystem does not write it into a hard disk drive, the server can execute the next processing promptly. However, even when the storage subsystem is provided with the cache, the improvement in speed by utilizing the cache cannot be achieved when the performance of the storage subsystem is limited by the read/write speed of the hard disk drive, that is, the storage subsystem performance is affected by the method by which the read/write processing of the hard disk drive is performed.
An example of the write (destage) processing algorithm into the hard disk drive from the cache in the conventional storage subsystem is a High/Low Mark Algorithm (see Anujan Varma and Quinn Jacobson, “Destage Algorithm for Disk Arrays with Non-Volatile Caches,” Proceeding of the 22nd International Symposium on Computer Architecture, pp. 83-95 (1995)). In the algorithm, when an amount of dirty data in the cache, which differs from the one stored in the hard disk drive caused by rewriting the data into the cache, exceeds a predetermined activation threshold value, a destage processing is activated, while the destage processing is terminated when the amount falls below a predetermined threshold value. Moreover, a similar one is the Linear Threshold Scheduling, where the data with the write cost lower than the upper limit, which will be modified according to the amount of dirty data, is destaged from the cache.
However, since such writing methods execute the destage processing based on the dirty data amount, when the write requests are gathered to the storage subsystem, the pace of destage processing must be changed in conjunction therewith. Furthermore, when the write operations are gathered to the hard disk drive due to the destage processing, it results in the decrease in readout performance from the hard disk drive. In order to prevent the decrease in read performance upon the write processing concentration, it is considered to apply the technology for scheduling the access by stabilizing a data transmission speed, as described in D. J. Gemmell et al, “Multimedia Storage Servers: A Tutorial,” IEEE Computer, pp. 40-49, May (1995). However, since the data transmission speed required for the general processing is unknown, the technology as described above cannot be applied directly for the destage control in a practical environment.
In the prior art described above, the practical scheduling of the internal access in the storage subsystem is not sufficiently realized, so that the read performance of the storage subsystem is decreased when the write requests are gathered to the storage subsystem. In view of the situation above, the present invention intends to prevent the decrease in performance of read access even when the write requests are gathered under a practical environment.